1. Technical Field
The present invention relates to a wavelength converter which is capable of obtaining a stable high output by combining a fiber laser and a wavelength conversion element.
2. Description of the Related Art
An intensely-monochromatic high-output visible light source is needed for realizing a large-sized display, a high-intensity display, or the like. Among the three primary colors of red, green and blue, in terms of a red light source, a red high-output semiconductor laser used for a DVD recorder or the like is available as a productive small-sized light source. However, a green or blue light source is difficult to realize in a semiconductor laser or the like, thus raising a demand for a productive small light source.
As such a light source, a wavelength converter obtained by combining a fiber laser and a wavelength conversion element is realized as a low-output visible light source. As a pumping light source which excites the fiber laser, a semiconductor laser is used, and a non-linear optical crystal is used as the wavelength conversion element. Such a green or blue light source is well known.
FIG. 25 shows a schematic configuration of a conventional wavelength converter 10. A laser beam from a fiber-pumping semiconductor laser 1 couples, through a lens 2, with an optical fiber 3 having a grating portion 6 embedded therein. Inside of this coupling, the laser beam goes back and forth repeatedly, so that an optical resonator is made up of the semiconductor laser 1 and the grating portion 6 of the optical fiber 3. This optical resonator emits a laser beam, and the laser beam is incident through a lens 4 upon a wavelength conversion element 5. Then, it is converted into a second harmonic wave and is emitted from the opposite plane to the incidence plane of the wavelength conversion element 5. At this time, the whole system's environment temperature changes, and the internal temperature of each member is raised. The wavelength band which can be converted by the wavelength conversion element 5 is approximately 0.1 nm, and this narrow band hinders the incidence light's wavelength from matching with the wavelength convertible by the wavelength conversion element 5. Hence, there is a disadvantage in that a stable output cannot be obtained from the wavelength conversion element 5.
In order to resolve this disadvantage, in an example shown in FIG. 25, a temperature-sensitive flexible member 7 which expands and contracts according to fluctuation in the temperature is expanded and contracted via a fixation portion 8 and a fixation portion 9 in the longitudinal directions of the grating portion 6 of the optical fiber 3. Thereby, the wavelength of light emitted from the optical resonator made up of the semiconductor laser 1 and the optical fiber 3 makes a change. Hence, even if the temperature change of the wavelength conversion element 5 varies the incident light's center wavelength convertible by the wavelength conversion element 5, that can be followed up. Such a follow-up system is presented (e.g., refer to Patent Document 1 and Patent Document 2).
Furthermore, using a temperature control circuit, the temperature of a polarization maintaining-type optical fiber is controlled so as to be kept constant. This makes it possible to obtain a stable output from a wavelength conversion element (e.g., refer to Patent Document 3).
Moreover, the temperature of a grating portion is detected by a temperature-sensitive element. Then, using a Peltier element, the temperature of a wavelength conversion element is controlled so that the wavelength conversion element's convertible wavelength band is included within the grating portion's reflection wavelength band at the detected temperature. Thereby, regardless of a change in the environment temperature or the like, a stable light output can be obtained from the wavelength conversion element (e.g., refer to Patent Document 4).
However, in the above described conventional wavelength converters, the wavelength variations according to the temperature change inside of such an optical-fiber grating portion and such a wavelength conversion element are 0.01 nm/K and 0.05 nm/K, respectively, different from each other. Therefore, if the internal temperature changes significantly, the wavelength selected in the grating portion goes far away from the wavelength convertible by the wavelength conversion element. When one tries to obtain low-output wavelength conversion light having an output of several hundred milli-watts or below, the methods according to the above described prior arts are effective. On the other hand, when one tries to obtain high-output wavelength conversion light of class W, particularly, the wavelength conversion element's internal temperature rises significantly to cause undue wavelength variation. As a result, in any method set forth in those prior arts, it will be difficult to adjust the temperature or the wavelength. This makes it hard to obtain a high class-W output.
In addition, the inventors of the present invention devotedly studied the destruction and degradation of a crystal which can be caused when a higher harmonic wave of several watts is generated. Consequently, the inventors found the cause of the destruction and degradation of a crystal in a principle totally different from any conventional optical damage. Hereinafter, the cause of such new crystal destruction and degradation will be described in detail.
A quasi phase-matching element (or QPM-LN element) formed by using Lithium Niobate crystal (or LN) or Lithium Tantalate (or LT) has a non-linear optical constant greater than an LBO crystal or a KTP crystal. This makes it possible to conduct an efficient and high-output wavelength conversion. In the QPM-LN element, however, optical energy has to be concentrated on a narrow area. Therefore, in practice, the destruction and degradation of a crystal by the fundamental wave or the generated second harmonic wave is more likely to occur than the KTP crystal.
In the case where a higher harmonic wave of several watts is obtained, the above described greater non-linear optical constant contributes to generating ultraviolet light (i.e., a third harmonic wave) which corresponds to the sum frequency of infrared light corresponding to the fundamental wave and green light (i.e., the second harmonic wave) obtained after the conversion, even if it does not meet a phase-matching condition. It was found out that this generated ultraviolet light triggers off absorption of the green light, thereby bringing about saturation of a green high output and crystal destruction. In this specification, the destruction of a crystal by this ultraviolet light (i.e., the third harmonic wave) is called crystal destruction by ultraviolet induced green-light absorption (or UVIGA), and thus, it is distinguished from the conventional optical damage.
FIG. 26 shows a measurement value and a theoretical value of the input-output characteristic of a conventional wavelength conversion element formed by using an LiNbO3 crystal including Mg added with 5.0 mol %. In this figure, the wavelength of a fundamental wave used in a measurement and a calculation is 1084 nm, and the element's length is 25 mm. The theoretical value is calculated using a method described in “T. Suhara and M. Fujimura: Waveguide Nonlinear-Optic Devices (Springer, Berlin, 2003)”, and a value which corresponds to each element is used for a conversion efficiency or the like.
As shown in FIG. 26, in the conventional wavelength conversion element formed by using the LiNbO3 crystal including Mg added with 5.0 mol %, the input-output characteristic for the theoretical value is indicated by a curve CR. The input is almost proportional to the output. On the other hand, the input-output characteristic for the measurement value is indicated by a curve CE. In an interval r1 where a green light output is below 1 W, the curve CR coincides almost with the curve CE. In contrast, in an interval r2 where the green light output is 1 W or above, the curve CE strays off the curve CR and the green light output becomes lower. Further, in an interval r3 where the green light output is 1.75 W or above, the curve CE strays far away from the curve CR and the green light output becomes unstable. As a result, in the conventional wavelength conversion element, it can be seen that if its output becomes equal to, or higher than, 1 W, then the ultraviolet induced green-light absorption is conspicuously caused.
As is varied according to elements, in the case where green light is generated, when an output of 1 W or above is generated, crystal destruction by the ultraviolet induced green-light absorption begins to occur. In the case where short-wavelength blue light is generated, the threshold of crystal destruction falls. If it becomes 0.1 W or above, crystal destruction by the ultraviolet induced green-light absorption begins to occur. In this way, the ultraviolet induced green-light absorption also causes undue wavelength variation and lowers the output of wavelength conversion light. This makes it difficult to obtain a high class-W output.
Patent Document 1: Japanese Patent Laid-Open No. 2004-165389 specification
Patent Document 2: Japanese Patent Laid-Open No. 2005-115192 specification
Patent Document 3: Japanese Patent Laid-Open No. 2005-181509 specification
Patent Document 4: Japanese Patent Laid-Open No. 2005-10340 specification